The present invention relates to method and apparatus for the production of a product by catalytic reaction of a pressurized synthesis gas. For example, one embodiment of the present invention relates to the production of ammonia by catalytic reaction of pressurized synthesis gas comprising hydrogen and nitrogen. More specifically, the present invention relates to an improved method for purification of make-up synthesis gas, i.e., synthesis gas which is added to the catalytic reactor to replace reacted synthesis gas.
U.S. Pat. No. 3,350,170 issued Oct. 31, 1967 to J. A. Finneran et al, discloses a process for carrying out cyclic synthesis reactions at elevated pressures and is particularly concerned with improvements in the method of compressing the fresh and recycle synthesis gases in such process. This patent well illustrates the type of synthesis process with which the present invention is concerned. As shown in FIG. 1 of U.S. Pat. No. 3,350,170, fresh synthesis gas 10 is introduced into a centrifugal compressor together with gas 42 recycled from a converter 38 in which hydrogen and nitrogen are catalytically converted to ammonia. The recycle gas exiting from converter 38 thus contains product ammonia as well as unreacted hydrogen and nitrogen. The recycle gas is reintroduced via line 24 into the compressor. The compressed outlet gas 26 thus comprises a mixture of the recycle gas plus the fresh (make-up) gas introduced via line 10. The product ammonia is separated in separation vessel 31 and the ammonia-depleted compressed synthesis gas travels to the converter 38 via lines 33, 34 and 35. Line 46 is used to separate a purged gas from the synthesis loop in order to prevent build-up of impurities in the synthesis loop defined by lines 42, 24, 26 and 33.
In conventional ammonia synthesis processes, removal of H2O from make-up synthesis gas is accomplished by mixing make-up gas containing about 160 ppm H2O with recycle gas at the compressor recycle wheel inlet. The gas discharged from the compressor is then cooled and chilled with H2O being absorbed in the condensing NH3. The NH3 and absorbed H2O are separated from the gas in a separator. The converter is fed with gas from the separator, which separated gas is substantially H2O-free or at least has only a very small residual H2O content. The separated gas may contain, for example, about 1.9% NH3. There are several disadvantages with this system. The refrigeration power required is higher because of the dilution of converter effluent with make-up gas that lowers the NH3 concentration and the dewpoint. This transfers load from the higher to the lower temperature chillers, which require more power per ton of refrigeration. Also, product NH3 is compressed in the recycle wheel, adding to the power demand imposed on the compressor. A significant improvement in reduced energy requirements can be realized for this system, as shown in U.S. Pat. No. 1,815,243, by incorporating a dehydrator.
A 1989 paper by H. Bendix and L. Lenz of VEB Agrochemie Piesteritz, the former German Democratic Republic (East Germany), was presented at a meeting of the American Institute of Chemical Engineers. The paper is entitled Results and Experiences on Revamping of Large-Scale Ammonia Single-Line Plants and discloses the addition, via a Venturi tube, of liquid ammonia to the synthesis gas discharged from the third stage of the synthesis gas compressor. The stated purpose is to dry the synthesis gas.
A paper by M. Badano and F. Zardi was presented at the 28 Feb.-2 Mar. 1999 Nitrogen ""99 meeting in Caracas, Venezuela sponsored by British Sulphur Publishing. The paper is entitled Casale Group Experience in Revamping Ammonia, Methanol and Urea Complexes and discloses scrubbing with liquid ammonia, ammonia synthesis gas between the second and third stages of the synthesis gas compressor.
Another prior art expedient is shown in U.S. Pat. No. 1,830,167 and Canadian Patent 257,043. This method involves scrubbing the combined make-up and recycle gas stream with liquid NH3 prior to preheating the stream and sending it to the converter. Normally, there is no need to scrub the recycle stream since there are no impurities in it. A drawback of the scheme of these patents is that it distributes impurities through the entire gas stream. It is then more difficult to effect complete impurity removal because the impurities are diluted by being dispersed throughout the entire gas stream. In order to treat the combined stream, the scrubbing apparatus must be much larger and more costly than would be required for scrubbing the makeup gas stream alone, since it is treating a gas volumetric flow which is 4-5 times greater than the make-up gas stream alone. Accordingly, the scheme of U.S. Pat. No. 1,815,243 and Canadian Patent 257,043 adds to the scrubbing load by combining the recycle and make-up streams prior to scrubbing.
Other prior art expedients include the use of molecular sieves to remove H2O from make-up gas by adsorption. The concept of dehydrating make-up gas permits the stream with the highest NH3 content, the effluent from the converter, to feed the chilling system. This saves considerable refrigeration power and can allow a significant capacity increase in plants that are limited by the size of the refrigeration compressor. The power savings is accomplished because of the elevated dew point that results in some condensation with cooling water and a transfer of load from the low to the high temperature chillers which need less power per ton of refrigeration. Removal of H2O by molecular sieves also enables omitting the purge gas chiller that uses the coldest NH3 refrigeration.
The H2O-free (and NH3-free) make-up gas is then mixed with recycle gas, compressed in the recycle wheel and fed to the converter. This system has one advantage over competing technologies, which is that the converter feed has a low NH3 content, about 1.4%. However, this advantage is offset by other factors such as the heat required for regeneration of the molecular sieves, the operating complexity because of the requirement for numerous switching valves needed for the cyclic operation to adsorb and desorb H2O from the molecular sieves, higher maintenance costs and the high capital cost of the molecular sieve vessels, heat exchangers, filters, piping and valves. The energy saving is estimated to be about 0.53 MM Btu/ST (where ST means short ton or 2000 pounds), compared to a standard secondary flash design.
Another prior art concept is shown in U.S. Pat. No. 3,349,569. This patent discloses installation of an NH3 scrubber at the inlet of the synthesis gas compressor, to use liquid NH3 to absorb H2O from make-up synthesis gas. This allows make-up gas to be mixed with the recycle gas and to be fed directly to the ammonia converter. The converter effluent then goes directly to a cooling/chilling system of the type described above in connection with the use of molecular sieves. A substantial chilling effect takes place because of the heat required to vaporize NH3, which comes from chilling the make-up gas. The essentially H2O-free make-up gas, which contains about 4.9% NH3, is then mixed with recycle gas as described above in connection with the use of molecular sieves.
There are several disadvantages with this system. Over-chilling of make-up gas due to excessive NH3 evaporation resulting from low pressure results in a scrubber overhead and compressor inlet temperature (xe2x88x9227xc2x0 F.) which is below the minimum (xe2x88x9220xc2x0 F.) for standard materials of construction. More expensive low-temperature materials of construction are needed for the scrubber, and the compressor will have to be re-rated (if possible). A re-rating of the compressor can sometimes be done if its original materials of construction were satisfactory for more severe operating conditions. Otherwise, an upgrade of the compressor low pressure case may be required and this is costly. Another disadvantage of this method is that NH3 will be contained in recycle gas sent to the front end of the plant for desulfurization, thereby lowering plant efficiency. This NH3 will be decomposed into H2 and N2 in the reforming section setting up a recycle loop. The suction scrubber is also at a disadvantage from a moisture removal standpoint since the equilibrium H2O content, although low, will be about two to three times higher than with the synthesis loop dehydrator of the present invention. The main disadvantage, however, of this prior art system stems from its low pressure operation and the resulting addition of a substantial quantity of NH3 to the converter feed gas which contains about 2.6% NH3. This reduces the energy savings potential to about 0.45 MM Btu/ST compared to a standard design with secondary flash (e.g. U.S. Pat. No. 1,815,243).
The version of the suction scrubber as described in U.S. Pat. No. 3,349,569 can use further cooling and chilling between compressor stages to condense some of the NH3 that was vaporized in the compressor inlet scrubber in the first place. The liquid NH3 formed serves to further purify the synthesis gas by random absorption of some of the remaining impurities. However, the refrigeration requirements of such a system would be prohibitive.
Yet another prior art system places the scrubber at the same pressure as the synthesis loop, i.e., about 1900 psia, which leads to its one advantage: minimizing the NH3 content in the scrubber overhead (2.7%) and in the converter feed (2.1%). There are, however, a number of disadvantages to this scheme. The most important one is the necessity to modify the second-stage case of the compressor in the case of a revamp of a 1900-2000 psia synthesis loop. A fourth nozzle must be added (a change that has never been done before) and the recycle wheel must be reduced in size. For the less common higher pressure loops (2500-3000 psia), the compressor second case already has four nozzles so addition of a nozzle is not an issue here. The risk involved with this type of modification of the compressor is substantial, since a number of problems (vibration, surge, oil leakage, bearing failure, etc.) can result. Further, the cost of the system for a retrofit is expected to be very high because of the compressor modification, the required addition of two more heat exchangers (scrubber inlet coolers) and the need for an NH3 pump. There is no compressor speed reduction as there is no NH3 evaporation and subsequent chilling for the make-up (first or second stages). Energy savings for a system with 36xc2x0 F. scrubber feed (avoiding a freezing problem) is expected to be about 0.44 MM Btu/ST.
Generally, the present invention provides a process and apparatus for producing ammonia from a pressurized synthesis gas comprising a mixture of hydrogen and nitrogen, which utilizes a dehydrator to remove H2O from the synthesis gas at an intermediate stage of the synthesis gas compressor.
In a preferred embodiment, the present invention provides for the use of substantially anhydrous liquid NH3 for scrubbing and subsequent cooling in the dehydrator of synthesis gas withdrawn between the first and second stages of a multi-stage compressor. This effects purification of the make-up gas and also reduces compression power requirements.
The present invention further integrates the improved purification step in the synthesis loop in such a way as to enhance the efficiency of the processing steps. Scrubbing make-up synthesis gas with liquid NH3 to remove impurities (mainly H2O) allows the synthesis gas to be mixed with recycle gas and fed directly to the converter. More specifically, purification of the make-up gas allows that gas to be mixed with NH3-lean gas for feeding the third or recycle stage of the compressor and then, the converter. Product NH3 is not compressed in the recycle wheel, which saves power. Converter effluent can be sent directly to the cooling/chilling system for NH3 condensation, thereby avoiding dilution with make-up gas and reducing refrigeration requirements. Power expenditure is thus reduced as compared to prior art systems. Product NH3 is removed prior to recycle compression.
The present invention, as compared to prior art schemes, reduces compression power requirements and process energy requirements, allows the option for raising plant capacity, reduces compressor speeds, operates the purification step (removal of H2O and other oxygenated impurities) at a pressure which is high enough to achieve sufficient purification without having to resort to further processing steps, and eliminates the prohibitively expensive compressor interstage refrigeration requirements required in some prior art schemes.
Specifically, in accordance with the present invention there is provided an improvement in a process for the manufacture of ammonia. The process comprises compressing in a multistage compressor a synthesis gas comprising hydrogen and nitrogen, each stage of the compressor having an inlet and a discharge associated therewith, contacting the compressed synthesis gas in an ammonia reactor with a suitable catalyst under conditions to promote the reaction of a portion, less than all, of the hydrogen and nitrogen in the synthesis gas to ammonia, separating product ammonia from a reactor effluent stream discharged from the ammonia converter, and recycling a portion of the reactor effluent stream containing unreacted hydrogen and nitrogen to the multi-stage compressor. The process includes withdrawing a make-up synthesis gas stream from the compressor and cooling and dehydrating the withdrawn synthesis gas stream, the dehydrating step being carried out by contacting the withdrawn synthesis gas stream with liquid ammonia, and returning the cooled and dehydrated synthesis gas stream to the compressor. The improvement comprises that the withdrawn synthesis gas stream is withdrawn from the discharge of the first stage of the compressor and returned to the compressor at the inlet of the second stage of the compressor.
Another aspect of the invention provides that the entire synthesis gas stream is withdrawn from the discharge of the first stage of the compressor and cooled and dehydrated.
In a specific aspect of the invention, the multi-stage compressor is a three-stage compressor and the synthesis gas is discharged from the first stage at a pressure of from about 800 to 900 psia, is discharged from the second stage of the compressor at a pressure of about 1800 to 1900 psia, and is discharged from the third stage of the compressor at a pressure of about 2000 to 2100 psia.
In one aspect of the invention, the withdrawn synthesis gas stream is cooled to a temperature of from about xe2x88x9220.5 to xe2x88x9226.1xc2x0 C. (xe2x88x925 to xe2x88x9215xc2x0 F.) prior to being returned to the compressor.
In another aspect of the present invention, the synthesis gas stream is returned to the compressor from the dehydrator without being warmed.
Another aspect of the invention provides that the H2O content of the withdrawn synthesis gas stream is reduced to less than 0.1 parts per million by volume prior to being returned to the compressor.
The invention also includes cooling the synthesis gas withdrawn from the compressor to condense ammonia contained therein and removing the condensed ammonia from the synthesis gas prior to introducing it into the ammonia converter.
The synthesis gas typically contains hydrogen and nitrogen in a molar ratio of about 3:1.
Yet another aspect of the invention provides an improvement in an apparatus for carrying out a process for the manufacture of ammonia by compressing in a multi-stage compressor having at least a first stage and a second stage a synthesis gas comprising hydrogen and nitrogen, each stage of the compressor having an inlet and a discharge associated therewith. The process comprises contacting the compressed synthesis gas in an ammonia reactor by contacting the compressed synthesis gas with a suitable catalyst under conditions to promote the reaction of a portion, less than all, of the hydrogen and nitrogen in the synthesis gas to ammonia and separating product ammonia from a reactor effluent stream discharged from the ammonia converter. The process further comprises recycling a portion of the reactor effluent stream containing unreacted hydrogen and nitrogen to the multi-stage compressor, and contacting the make-up synthesis gas with liquid ammonia in a dehydrator having a synthesis gas inlet, a synthesis gas outlet and a liquid ammonia inlet and a liquid ammonia outlet. The improvement to the apparatus comprises that the compressor is fitted with (a) a synthesis gas outlet connecting in flow communication the discharge of the first stage with the synthesis gas inlet of the dehydrator, and (b) a synthesis gas intermediate inlet connecting the inlet of the second stage in flow communication with the synthesis gas outlet of the dehydrator, whereby to define a synthesis gas flow path from the discharge of the first stage, through the dehydrator, thence to the inlet of the second stage.
The synthesis gas and liquid ammonia inlets and outlets are preferably arranged to flow the liquid ammonia countercurrently to the synthesis gas in the dehydrator.
An apparatus aspect of the present invention provides that the apparatus further comprises a heat exchanger to cool the synthesis gas and a liquid-vapor separator to separate H2O therefrom, the heat exchanger and liquid-vapor separator being disposed in the synthesis gas flow path between the first stage of the compressor and the synthesis gas inlet of the dehydrator.